docs: document S3 backend, auto-auth, and :Pending done command (#125)

Problem: The S3 backend had no `:Pending s3` entry in the COMMANDS
section, `:Pending auth` only mentioned Google, the `sync` config
field omitted `s3`, `_s3_sync_id` was missing from the data format
section, `:Pending done` was implemented but undocumented, and the
README lacked a features overview.

Solution: Add `:Pending s3` and `:Pending done` command docs, rewrite
`:Pending auth` to cover all backends and sub-actions, update config
and data format references, add `aws` CLI to requirements, and add a
Features section to `README.md`.

README.md

@@ -8,10 +8,27 @@ Oil-like task management for todos in Neovim, inspired by
 
 https://github.com/user-attachments/assets/f3898ecb-ec95-43fe-a71f-9c9f49628ba9
 
+## Features
+
+- Oil-style buffer editing: standard Vim motions for all task operations
+- Inline metadata: `due:`, `cat:`, `rec:` tokens parsed on `:w`
+- Rich date input: relative (`+3d`, `tomorrow`), weekdays, ordinals, custom formats, time suffixes
+- Recurring tasks with automatic next-date spawning on completion
+- Category and queue views with foldable sections
+- Multi-level undo (up to 20 saves, persisted across sessions)
+- Text objects (`at`/`it`/`aC`/`iC`) and motions (`]]`/`[[`/`]t`/`[t`)
+- Omnifunc completion for `due:`, `cat:`, and `rec:` tokens
+- Filters: `cat:X`, `overdue`, `today`, `priority`, `wip`, `blocked`
+- Google Calendar one-way push via OAuth PKCE
+- Google Tasks bidirectional sync via OAuth PKCE
+- S3 whole-store sync via AWS CLI with cross-device merge
+- Auto-authentication: sync actions trigger auth flows automatically
+
 ## Requirements
 
 - Neovim 0.10+
-- (Optionally) `curl` for Google Calendar and Google Task sync
+- (Optionally) `curl` for Google Calendar and Google Tasks sync
+- (Optionally) `aws` CLI for S3 sync
 
 ## Installation